1. Field of the Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for providing high voltages for erasing and programming memory arrays from either an internal or an external source of voltage.
2. History of the Prior Art
It has been found that the use of computers has grown so extensive that the power used by these computers has become significant. In order to reduce the cost of operation as well as the consequent use of eneray resources, a substantial move is underway to reduce this power usage. In fact, a major trend in the manufacture of personal computers is to reduce the voltage level required to operate the integrated circuits which are used in the various components of those computers. There has also been a simultaneous trend to provide portable computers which function for extended periods. This has also has led to attempts to reduce the power used by portable computers.
In order to reduce power consumption and extend battery life, much of the integrated circuitry used in personal computers is being redesigned to run at low voltage levels. This reduces the power usage and allows more components to be placed closer to one another in the circuitry. The circuitry and components used in portable computers are being designed to operate at voltages levels such as five volts and 3.3 volts. This helps a great deal to reduce the power needs of personal computers.
However, at the same time, the desire to offer more features in portable computers opposes this salutary result. Many of these features require higher voltages to function. For example, one real convenience is the ability to change the basic input/output (BIOS) processes as improvements in a computer or its peripherals occur. Historically, this has been accomplished by removing the electrically programmable read only memory (EPROM) or similar circuitry which provides the read only memory for storing the BIOS processes and replacing it with new circuitry at additional cost. This is a complicated operation beyond the abilities of many computer users. Recently, flash electrically-erasable programmable read only memory (flash EEPROM memory) has been used to store BIOS processes. This memory may be erased and reprogrammed without removing the BIOS circuitry from the computer by running a small update program when the BIOS processes are changed. However, reprogramming flash EEPROM memory requires approximately twelve volts to accomplish effectively; and the lower voltage batteries provided in personal computers are not capable of furnishing this voltage.
Another type of flash EEPROM memory array provides an additional example of high voltage requirements in portable computers. Recently, a new form of long term random access storage has been devised using flash EEPROM memory arrays. An example of a flash EEPROM memory array which may be used in place of a hard disk drive is given in U.S. patent application Ser. No. 07/969,131, entitled A Method and Circuitry For A Solid State Memory Disk, S. Wells, filed Oct. 31, 1992, and assigned to the assignee of the present invention. These arrays provide a smaller lighter functional equivalent of a hard disk drive which operates more rapidly and is not as sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is important.
However, these flash EEPROM memory arrays also require much higher voltages for programming and erasing data than can be provided directly by the batteries of portable computers. Although it has been typical to provide charge pumps to generate higher voltage in circuitry in which batteries do not provide sufficient voltages, this has not been possible with flash EEPROM memory arrays. Now a new arrangement has been devised by which charge pump circuitry may be integrated with a flash EEPROM memory array and used to generate the high voltages necessary for programming and erasing such an array. A detailed description of one such charge pump is furnished in U.S. patent application Ser. No. 08/119,423, entitled Method and Apparatus for A Two Phase Bootstrap Charge Pump, K. Tedrow et al, filed on even date herewith, and assigned to the assignee of the present invention.
It has been found to be very desirable that the circuitry by which erasing and programming of flash EEPROM memory arrays is accomplished be capable of utilizing erasing and programming voltages either generated internally by charge pump circuitry from a low voltage level external source or, alternatively, provided externally from a high voltage level source.